HVDC power systems are connected to AC networks by converter stations which convert between DC on the HVDC side and AC on the AC network side. SCADA (supervisory control and data acquisition) is a type of computer-controlled industrial control system that monitors and controls operation of such an integrated HVDC/AC network. SCADA typically runs programs such as OPF (optimal power flow) and provides each of the converter stations a P-Q (real and reactive power) set point once every five to fifteen minutes. An HLC (high level controller) which controls operation of the HVDC system may communicate and provide new set points to the converter stations more frequently than the SCADA system performs OPF, e.g. every 1-60 seconds. As such the HLC can change the P-Q set points of the converter stations during this inter-OPF period. However, the HLC must ensure that the new P-Q set points do not compromise the security or reliability of the AC networks. Otherwise, faults or other undesirable conditions or actions may occur in the AC networks. For example, bus voltage limits, line current limits and/or line thermal limits may be exceeded in the AC networks in response to particular changes in the P-Q set points of the converter stations.